1. Filed of the Invention
The present invention relates to an optical transceiver that includes an optical transmitter and an optical receiver built in single package.
2. Related Prior Art
In the optical transceiver that includes both the optical transmitter and the optical receiver within the same housing, the noise attributed to the optical transmitter influences the performance of the optical receiver, which is the so-called crosstalk. To suppress this crosstalk is generally used an EMI shield technique in the conventional transceiver.
However, it has been unsuccessful to reduce the noise from the optical transmitter to a level not affecting on the performance of the optical receiver. Several techniques have been known in a field of the optical communication. Japanese patent published as 2002-335215 has disclosed one technique, in which the threshold level for discriminating the data involved in the input optical signal is dynamically adjusted with respect to the transmitted signal that has a burst mode. When the burst mode is ON, i.e., data being practically transmitted, the threshold level is set to the first level, while the burst mode is OFF, i.e., data transmitting being in rest, the threshold level is set to the second level.
The U.S. Pat. No. 5,652,425 has disclosed another technique, in which a receiving optical subassembly that generally includes a light-receiving device and a preamplifier for converting a photocurrent generated by the light-receiving device into a corresponding electrical signal. The subassembly in this prior art further includes a dummy capacitor, the capacitance of which is equivalent to that of the light-receiving device, and a dummy amplifier connected to the dummy capacitor. By connecting the these dummy capacitor and the dummy amplifier to the same power supply with that of the light-receiving device and the preamplifier, and feeding the outputs of the preamplifier and the dummy amplifier by a differential circuit, the common mode noise attributed to the fluctuation of the power supply and the ground line may be cancelled at the output of the differential amplifier.
FIG. 8 shows a configuration that combines the first prior art, JP 2002-335215, with the second prior art, U.S. Pat. No. 5,652,425. This optical transceiver comprises the light-receiving device 1, the dummy device that corresponds to the dummy capacitor in the U.S. Pat. No. 5,652,425, the preamplifier 3, the dummy amplifier 4, the differential circuit 5, the main amplifier 6 for the optical receiver, while the optical transmitter includes the light-emitting device 7, the driver 8, and the threshold generator 9.
In the optical receiver, the light-receiving device 1, the dummy device 2, the preamplifier 3, the dummy amplifier 4, and the differential amplifier 5 are installed within a housing. By feeding respective outputs of two amplifiers to the non-inverting and inverting inputs of the differential amplifier 5, the common mode noise superposed on the outputs of two amplifiers, 3 and 4, may be cancelled.
Moreover, an electrical output from the driver 8 in the optical transmitter is fed in the threshold generator 9, in which the output thereof may be adjusted depending on the switching status of the driver 8. This configuration from the output of the driver 8 to the main amplifier 6 in the optical receiver via the threshold generator 9 follows the JP 2002-335215.
In the optical transceiver, typically shown in FIG. 8, the light-emitting device such as laser diode (LD) is modulated by comparably large current over 10 mA. When such large current is switched, a noise is generated by the electromagnetic radiation. Moreover, such large current causes a fluctuation in the power supply line and the ground line due to the equivalent resistance of these lines. Although the power supply line or the ground line are considered to be 0 Ω in the circuit diagram, these lines practically shows substantial resistance. In particular, when the lines are formed by a thin metal film such as on a printed circuit board, the equivalent resistance can not be ignored. To flows a large current in such lines with substantial resistance causes the fluctuation in the voltage thereof, and this fluctuation may be transmitted to the optical receiver as a noise.
On the other hand in the optical receiver, a faint optical signal is necessary to be converted into a corresponding electrical signal and to be amplified by a level capable of being processed in the subsequent stage of the receiver. Generally, the total gain reaches around 30 dB or 40 dB. Accordingly, the noise attributed to the fluctuation of the power lines and the ground lines is also amplified by the gain above. This noise is typically called as the common mode noise.
The differential amplifier may process two signals complementary to each other, namely, having phases different by 180° with respect to the other. This differential amplifier is generally used in the circuit for the small signal because the signal level may be equivalently expanded by twice, which enhances the tolerance to the noise. That is, when one input of the differential amplifier receives a signal with the positive phase, the other input thereof receives another signal with the negative phase at the same time, which equivalently magnifies the input level. Moreover, even the power lines fluctuate by the reason explained above, this fluctuation affects to both signals with the positive and negative phases. That is, assuming that the fluctuation is δ, the signal with the positive phase becomes sig+δ, while that with the negative phase becomes /sig+δ. Here, sig and /sig denote the positive and negative signals, respectively. Finally, the output of the differential amplifier becomes;A*{(sig+δ)−(/sig+δ)}=A*(sig−/sig),where Ais the gain of the amplifier. Thus, the fluctuation δ is not reflected on the output of the differential amplifier.
The prior art shown in FIG. 8, according to the explanation above, provides the dummy device 2 and the dummy amplifier 4 in the receiving optical subassembly, and the differential amplifier to eliminate the common mode noise mentioned above. The light-receiving device 1 and the preamplifier 3 are influenced by the noise via the power supply line and the ground line, while the dummy device 2 and the dummy amplifier 4 are also influenced by the same noise. Receiving two outputs from respective amplifiers by the differential amplifier 5, this common mode noise can be eliminated at the output of the differential amplifier 5, which enhances the noise tolerance of the optical transceiver.
The circuit shown in FIG. 8 may eliminate the noise influencing the light-receiving device 1 and the preamplifier 3. However, for the noise superposed on the signal line between the receiving subassembly and the main amplifier 6, the conventional circuit of FIG. 8 lacks its effect. Therefore, the main amplifier 6 receives the compensation signal form the threshold generator 9 to cancel the noise superposed on the signal line from the differential amplifier 5.
However, the threshold generator 9 traces the bust mode of the transmitted signal as disclosed in JP 2002-335215, which may respond to a relatively slow signal and may not trace the transmitted signal in a bit-by-bit mode. It is quite hard to vary the threshold of the main amplifier in the bit-by-bit mode corresponding to the transmitted signal by the threshold generator 9. Moreover, the circuit shown in FIG. 9 feeds the input of the threshold generator 9 from the output of the driver 8, which may degrade the optical output in the high frequency.
Thus, the present invention is to provide an optical transceiver that effectively eliminates the noise generated by the current switching in the optical transmitter and superposed on the signal line of the optical receiver, accordingly, to provide an optical transceiver with a improved receiving sensitivity.